The cpn60 protein of B. pertussis, the major molecular chaperone produced by this organism was studied. This protein has the tetradecameric structure typical of the GroEL family of proteins. Studies were conducted to examine the structure/function relationships of cpn60 in the hope of better understanding the role that this protein plays in protein folding. An IgG monoclonal antibody (mAb 54G8) which binds to both B. pertussis cpn60 and Escherichia coli cpn60 (GroEL) was found to abolish the ability of cpn10 to inhibit the ATPase activity cpn60 regardless of the source of this protein. MAb 54G8 was shown by electron microscopy to bind to ends of the tetradecameric cpn60 molecules. Computer image processing of electron micrographs of the cpn60-Ab was performed. Averages of cpn60-Ab complex from individual side views shown centers of mass attached to and extending from the corners of either hepameric face of the cpn60 molecule. No more than two antibodies appear to bind at each end of cpn60. Averages of electron micrographs of cpn60 molecules interacting with only the Fab portion of the antibody show to be perturbed when interacting with Fab fragments. Our data suggest that binding of mAb 54G8 at the end of cpn60 partially affects the ability of cpn60 productively interact with cpn10.